Semiconductor device with under bump metallurgy and method for fabricating the same

ABSTRACT

A semiconductor device with under bump metallurgy (UBM) and a method for fabricating the semiconductor device are provided, wherein a passivation layer is deposited on a surface of the semiconductor device where a plurality of bond pads are disposed, and formed with a plurality of openings for exposing the bond pads. A first metal layer is deposited over part of each of the bond pads and a portion of the passivation layer around the bond pad; then, a second metal layer is formed over the first metal layer and part of the bond pad uncovered by the first metal layer; subsequently, a third metal layer is formed over the second metal layer to thereby fabricate a UBM structure. Finally, a solder bump is formed on the UBM structure so as to achieve good bondability and electrical connection between the solder bump and UBM structure.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices with under bumpmetallurgy (UBM) and fabrication methods thereof, and more particularly,to a flip-chip semiconductor device formed with UBM structures and amethod for fabricating the semiconductor device.

BACKGROUND OF THE INVENTION

Flip-chip semiconductor packages employ advanced packaging technologythat is characterized by mounting a semiconductor chip in a face-downmanner on a substrate and electrically connecting the semiconductor chipto the substrate via a plurality of solder bumps. This structure yieldssignificant benefits without having to use relatively space-occupyingbonding wires for electrically connecting the semiconductor chip to thesubstrate, thereby making the overall package structure more compact insize.

Referring to FIG. 1, for forming a solder bump 150 to a semiconductorchip 100, the first step is to form an under bump metallurgy (UBM)structure 130 on a bond pad 110 of the semiconductor chip 100. The UBMstructure 130 includes an adhesion layer 130 a such as aluminum layerformed over the bond pad 110; a barrier layer 130 b such asnickel/vanadium (Ni/V) alloy applied over the adhesion layer 130 a; anda wetting layer 130 c such as copper layer formed on the barrier layer130 b. A solder material can be applied over the wetting layer 130 c andreflowed to form the solder bump 150 on the UBM structure 130. This UBMstructure 130 serves as a diffusion barrier and provides proper adhesionbetween the solder bump 150 and the bond pad 110 of the semiconductorchip 100.

Fabrication of the UBM structure generally adopts sputtering,evaporation and plating processes.

FIGS. 2A to 2E illustrate conventional fabrication processes for asolder bump on a flip chip. Referring to FIG. 2A, the first step is toprepare a semiconductor chip 100 formed with a plurality of bond pads110 on a surface thereof, and to apply a passivation layer 120 over thesurface of the semiconductor chip 100. The passivation layer 120 isselectively removed to expose the bond pads 110 of the semiconductorchip 100. Then, sputtering and plating processes are performed to form aUBM structure 130 on each of the bond pads 110.

Referring to FIG. 2B, next, a solder mask film 140 such as dry film isapplied over the passivation layer 120 and formed with a plurality ofopenings 141 for exposing the UBM structures 130.

Referring to FIG. 2C, then a solder-applying process is performed bywhich a solder paste such as tin/lead (Sn/Pb) alloy is applied via theopenings 141 through the use of screen-printing technology over the UBMstructures 130 to form a plurality of solder bumps 150 respectively onthe UBM structures 130.

Referring to FIG. 2D, a first reflow process is carried out to bond thesolder bumps 150 to the corresponding UBM structures 130. Then, thesolder mask layer 140 is removed, and a second reflow process isperformed to make the solder bumps 150 have a ball shape, as shown inFIG. 2E.

Prior art references relating to UBM technology include, for example,U.S. Pat. No. 5,773,359 entitled “Interconnect System and Method ofFabrication”, U.S. Pat. No. 5,904,859 entitled “Flip ChipMetallization”, and U.S. Pat. No. 5,937,320 entitled “Barrier Layers forElectroplated SnPb Eutectic Solder Joints”; to name just a few.

In respect of fabricating a UBM structure on an aluminum-made bond pad(hereinafter referred to as “aluminum pad”) of a semiconductor chip, analuminum layer (or a chromium layer) is firstly formed over the aluminumpad to provide adhesion between the aluminum pad and the UBM structure.Then, a nickel/vanadium (Ni/V) layer is deposited over the aluminumlayer to serve as a barrier for preventing intermetallic compoundsformed from reaction between the aluminum pad and a solder-bumpelectrode. Finally, a copper layer (or a layer made of nickel, palladiumor molybdenum) is applied on the Ni/V layer for allowing the solder bumpto be successfully bonded to the UBM structure. However, this UBMstructure is not applicable to a copper-made bond pad (hereinafterreferred to as “copper pad”) because the aluminum layer formed over thebond pad has relatively poor adhesion to copper, making the UBMstructure not strongly bonded to the copper pad. Therefore, for forminga UBM structure on a copper pad, the first step is to apply a titanium(Ti) layer over the copper pad, and the Ti layer provides good adhesionbetween the UBM structure and the copper pad. Then, a Ni/V layer and acopper layer are formed over the Ti layer for allowing a solder bump tobe strongly bonded thereon. Although the Ti layer enhances adhesionbetween the copper pad and the UBM structure, this Ti layer is poorer inelectrical conductivity than aluminum and thus degrades electricalconnection between the solder bump and the copper pad.

Therefore, the problem to be solved herein is to provide good electricalconnection and adhesion between a UBM structure and a copper-made bondpad.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide asemiconductor device with under bump metallurgy (UBM) and a method forfabricating the same, so as to effectively improve electrical connectionbetween a UBM structure and a bond pad of the semiconductor device.

Another objective of the invention is to provide a semiconductor devicewith under bump metallurgy and a method for fabricating the same, whichcan greatly enhance adhesion between a UBM structure and a bond pad or apassivation layer of the semiconductor device.

In accordance with the foregoing and other objectives, the presentinvention proposes a semiconductor device with under bump metallurgy(UBM), comprising: a device body having at least a surface formed with aplurality of bond pads thereon; a passivation layer applied over thesurface of the device body and formed with a plurality of openings forexposing the bond pads via the openings; a plurality of UBM structures,each of which includes a first metal layer for covering part of each ofthe bond pads and a portion of the passivation layer around thecorresponding bond pad, a second metal layer for covering the firstmetal layer and part of the bond pad uncovered by the first metal layer,and at least a third metal layer applied over the second metal layer;and a plurality of solder bumps formed on the UBM structuresrespectively.

A method for fabricating the semiconductor device with under bumpmetallurgy of the invention comprises the steps of: applying apassivation layer over at least a surface of a device body formed with aplurality of bond pads thereon, wherein the passivation layer is formedwith a plurality of openings for exposing the bond pads via theopenings; forming a first metal layer (such as a titanium layer) overpart of each of the bond pads and a portion of the passivation layeraround the corresponding bond pad; applying a second metal layer (suchas a copper layer) over the first metal layer and part of the bond paduncovered by the first metal layer; applying at least a third metallayer (such as a nickel layer) over the second metal layer; and forminga solder bump on the UBM structure.

The above semiconductor device with under bump metallurgy (UBM) ischaracterized in that the first metal layer (titanium layer) of the UBMstructure only covers partly the bond pad and the passivation layeraround the bond pad, thereby assuring good adhesion between the UBMstructure and the bond pad and passivation layer. Moreover, with thebond pad being only partly covered by the first metal layer, the secondmetal layer (copper layer) subsequently formed on the first metal candirectly come into contact with the copper-made bond pad, so as toprovide good electrical connection between the UBM structure and thebond pad.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thefollowing detailed description of the preferred embodiments, withreference made to the accompanying drawings, wherein:

FIG. 1 (PRIOR ART) is a cross-sectional view of a conventional underbump metallurgy (UBM) structure;

FIGS. 2A-2E (PRIOR ART) are cross-sectional diagrams showingconventional fabrication processes for forming a solder bump on a flipchip;

FIG. 3A is a cross-sectional view of a semiconductor device with underbump metallurgy according to a first preferred embodiment of theinvention;

FIGS. 3B and 3C are each a top view of a first metal layer formed in thesemiconductor device with under bump metallurgy shown in FIG. 3A;

FIG. 4A is a cross-sectional view of a semiconductor device with underbump metallurgy according to a second preferred embodiment of theinvention;

FIGS. 4B and 4C are each a top view of a first metal layer formed in thesemiconductor device with under bump metallurgy shown in FIG. 4A; and

FIGS. 5A-5E are cross-sectional diagrams showing fabrication processesfor fabricating the semiconductor device with under bump metallurgyaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3A illustrates a cross-sectional view of a semiconductor devicewith under bump metallurgy (UBM) according to a first preferredembodiment of the present invention.

This semiconductor device is composed of a device body 200 formed with aplurality of bond pads 210 on at least a surface thereof; a passivationlayer 220 applied over the surface of the device body 200 and formedwith a plurality of openings for exposing the bond pads 210 via theopenings; a plurality of UBM structures 230 respectively formed on theexposed bond pads 210, wherein each of the UBM structures 230 includes afirst metal layer 230 a for covering part of the corresponding bond pad210 and a portion of the passivation layer 220 around the bond pad 210,a second metal layer 230 b for covering the first metal layer 230 a andpart of the bond pad 210 uncovered by the first metal layer 230 a, andat least a third metal layer 230 c applied over the second metal layer230 b; and a plurality of solder bumps 240 formed on the UBM structures230 respectively.

The device body 200 can be a substrate used in a semiconductor package,or a printed circuit board (PCB) for accommodating electronic elements;preferably, the device body 200 is a semiconductor chip or wafer for aflip-chip structure. The plurality of bond pads 210 formed on the devicebody 200 can be e.g. copper pads acting as input/output (I/O) pads ofthe device body 200. For the sake of simplicity, only one bond pad isillustrated in the drawings for the following description.

The passivation layer 220 is a dielectric layer generally made ofpolyimide, silicon dioxide, silicon nitride, etc. The passivation layer220 covers the surface of the device body 200 formed with the bond pads210 and protects the surface against external contamination and damage.Moreover, the passivation layer 220 is formed with a plurality ofopenings for exposing the bond pads 210 via the openings.

The UBM structure 230 is composed of a plurality of metal layers formedon each of the bond pads 210. For use with a copper-made bond pad 210,first, a first metal layer 230 a such as a titanium (Ti) layer isapplied over the bond pad 210 and provides good adhesion between the UBMstructure 230 and the bond pad 210. This first metal layer 230 a coversa central portion of the bond pad 210 and a portion of the passivationlayer 220 around the bond pad 210, making an annular portion of the bondpad 210 and a side edge of the passivation layer 220 around the bond pad210 exposed to outside of the first metal layer 230 a, as shown in FIG.3B. Moreover, for enhancing adhesion between the first metal layer 230 aand the bond pad 210 or passivation layer 220, as shown in FIG. 3C, atleast a connecting portion can be formed to interconnect portions of thefirst metal layer 230 a respectively covering the central portion of thebond pad 210 and the portion of the passivation layer 220; thisconnecting portion is not particularly limited in metal material formaking the same and in shape or quantity thereof.

The UBM structure 230 also includes a second metal layer 230 b such as acopper layer, which is formed over the first metal layer 230 a forcovering the first metal layer 230 a and the exposed annular portion ofthe bond pad 210, wherein the second metal layer (copper layer) 230 bdirectly comes into contact with the copper-made bond pad 210 to therebyprovide good electrical connection between the UBM structure 230 and thebond pad 210. The UBM structure 230 may further include a third metallayer 230 a such as a nickel layer formed over the second metal layer230 b. This third metal layer 230 c provides satisfactory protection forthe second metal layer 230 b and good wetting effect for a solder bump240 subsequently bonded to the third metal layer 230 c, so as to allowthe solder bump 240 to be well connected to the UBM structure 230.

FIG. 4A illustrates a semiconductor device with under bump metallurgyaccording to a second preferred embodiment of the invention. As shown inthe drawing, this semiconductor device is structurally similar to butdiffers from the above semiconductor device of the first preferredembodiment in that, in the second embodiment, the first metal layer(titanium layer) 230 a covers the central portion of the bond pad 210and the portion and side edge of the passivation layer 220 around thebond pad 210, only allowing the annular portion of the bond pad 210 tobe exposed, as shown in FIG. 4B. This increases applied area of thefirst metal layer 230 a to thereby enhance adhesion between the UBMstructure 230 and the bond pad 210 and passivation layer 220. Moreover,at least a connecting portion of the first metal layer 230 a can beformed across the exposed annular portion of the bond pad 210, as shownin FIG. 4C.

FIGS. 5A to 5E illustrate fabrication processes for fabricating thesemiconductor device with under bump metallurgy according to the secondpreferred embodiment of the invention.

Referring to FIG. 5A, the first step is to apply a passivation layer 320such as a silicon-dioxide layer over a surface of a semiconductor chip300 formed with a plurality of copper-made bond pads 310 thereon,wherein the passivation layer 320 is formed with a plurality of openings321 for exposing the bond pads 310 via the openings 321.

Referring to FIG. 5B, the next step is to perform a sputtering processto form a first metal layer 330 a such as a titanium layer of a UBMstructure 330 over the passivation layer 320 and the exposed bond pads310.

Referring to FIG. 5C, an etching process is performed to selectivelyremove the first metal layer 330 a, so as to form a pattern of the firstmetal layer 330 a (as shown in FIG. 4B) that covers part of each of thebond pads 310 and a portion of the passivation layer 320 around thecorresponding bond pad 310. By good adhesion between the first metallayer (titanium layer) 330 a and the copper-made bond pad 310, itassures satisfaction connection between the UBM structure 330 and thebond pad 310.

Referring to FIG. 5D, after completing patterning of the first metallayer 330 a, a second metal layer (such as a copper layer) 330 b and athird metal layer (such as a nickel layer 330 c) are plated over thefirst metal layer 330 a. The second metal layer 330 b covers the firstmetal layer 330 a and part of the bond pad 310 uncovered by the firstmetal layer 330 a, allowing the second metal layer 330 b to be in directcontact with the copper-made bond pad 310. Then, the third metal layer330 c is formed over the second metal layer 330 b, and completesfabrication of the UBM structure 330.

Referring to FIG. 5E, finally a solder material is applied over thethird metal layer 330 c and reflowed to form a solder bump 340 on theUBM structure 330. This thereby completes fabrication of thesemiconductor device with under bump metallurgy (UBM).

The above UBM structure provides significant benefits. The first metallayer (titanium layer) partly covers both the bond pad and thepassivation layer around the bond pad, thereby assuring good adhesionbetween the UBM structure and the bond pad and passivation layer.Moreover, with the bond pad being only partly covered by the first metallayer, the second metal layer (copper layer) subsequently formed on thefirst metal can directly come into contact with the copper-made bondpad, so as to provide good electrical connection between the UBMstructure and the bond pad.

The invention has been described using exemplary preferred embodiments.However, it is to be understood that the scope of the invention is notlimited to the disclosed embodiments. On the contrary, it is intended tocover various modifications and similar arrangements. The scope of theclaims, therefore, should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1-9. (canceled)
 10. A method for fabricating a semiconductor device withunder bump metallurgy, comprising the steps of: applying a passivationlayer over at least a surface of a device body formed with a pluralityof bond pads thereon, wherein the passivation layer is formed with aplurality of openings for exposing the bond pads via the openings;forming an under bump metallurgy (UBM) structure on each of the bondpads, the UBM structure comprising a plurality of metal layers, whereina first metal layer covers part of each of the bond pads and a portionof the passivation layer around the corresponding bond pad, and a secondmetal layer of the UBM structure covers the first metal layer and partof the bond pad uncovered by the first metal layer; and forming a solderbump on the UBM structure.
 11. The method for fabricating asemiconductor device with under bump metallurgy of claim 10, wherein theUBM structure further comprises at least a third metal layer formed overthe second metal layer.
 12. The method for fabricating a semiconductordevice with under bump metallurgy of claim 10, wherein the device bodyis selected from the group consisting of a semiconductor chip, wafer,substrate and circuit board.
 13. The method for fabricating asemiconductor device with under bump metallurgy of claim 10, wherein thebond pads are made of copper.
 14. The method for fabricating asemiconductor device with under bump metallurgy of claim 10, wherein thefirst metal layer is made of titanium.
 15. The method for fabricating asemiconductor device with under bump metallurgy of claim 10, wherein thesecond metal layer is made of copper.
 16. The method for fabricating asemiconductor device with under bump metallurgy of claim 11, wherein thethird metal layer is made of nickel.
 17. The method for fabricating asemiconductor device with under bump metallurgy of claim 10, wherein thepassivation layer is a dielectric layer.
 18. The method for fabricatinga semiconductor device with under bump metallurgy of claim 17, whereinthe passivation layer is made of a material selected from the groupconsisting of polyimide, silicon dioxide and silicon nitride.